1. Field of the Invention
This invention relates to gain-producing fibers (GPFs) that support either a single signal mode or few signal modes and, more particularly, to double-clad GPFs designed for increased cladding absorption of pump light while maintaining operation in a single signal mode.
2. Discussion of the Related Art
Single-mode GPFs with a double-clad fiber (DCF) design are commonly used in high power optical fiber lasers and amplifiers requiring good beam quality. FIG. 7 illustrates a well known DCF 70, which comprises a core region 70.1, an inner cladding region 70.3 surrounding the core region, and an outer cladding region 70.4 surrounding the inner cladding region. The waveguide formed by the core and inner cladding regions primarily is configured to support and guide the propagation of signal light in a single mode; that is, preferably in the fundamental (LP01) mode.
In order to produce gain when suitably pumped, the core region is doped with gain-producing species, usually one or more rare-earth elements (e.g., Er, Yb, Tm, Nd) or one or more non-rare-earth elements (e.g., Cr, Bi) depending on the wavelength of the signal light to be amplified or the laser light to be generated. Multi-mode pump light coupled (launched) into the core region via (through) the inner cladding region reflects from the interface 70.5 between the inner and outer cladding regions, and as it propagates down the fiber axis, pump light traverses the core region where it is absorbed by particular dopants (i.e., gain-producing species) in the core region. In this fashion, pump light energy amplifies signal light that is simultaneously propagating down (predominantly) the core region of the fiber.
The effectiveness of the amplification process (i.e., the energy transfer from pump light to signal light) is determined, in part, by a parameter (αclad) known as cladding absorption (of pump light) given byαclad=αd[Ad/Aclad].  (1)where αd is the material absorption of pump light in the portion of the fiber doped with gain-producing species (hereinafter the gain region; e.g., the core region 70.1 of FIG. 7), Ad is the transverse cross-sectional area of the gain region, and Aclad is the entire transverse cross-sectional area within the inner cladding region 70.3 (e.g., πDic2/4 for an inner cladding region having a circular cross-section). In turn, material absorption is given byαd=Ndσd,  (2)where Nd is the volume concentration of gain-producing species in the gain region, and σd is the absorption cross-section of the dopants in the gain region matrix (e.g., crystal or glass) at the pump light wavelength.
The ability to increase cladding absorption of pump light would be advantageous. In fiber optic lasers and amplifiers, for a given output power, increased pump light absorption implies increased gain, which, in turn, means that the desired output power can be achieved with shorter fiber lengths in amplifiers and shorter cavity resonator lengths in lasers. Shorter fiber lengths, in turn, reduce the onset of nonlinear effects such as stimulated Raman scattering (SRS) and may be beneficial to improving fiber laser power stability and long term reliability.
Equations (1) and (2) would seem to suggest that cladding absorption could be increased by simply increasing Nd, the concentration of gain-producing species in the core region 70.1, or more precisely in the doped region Ad, of the fiber. However, for certain commercially important rare-earth species (Yb in particular) pump-light-induced photodarkening limits the concentration of the gain-producing species, thereby limiting the cladding absorption that can be achieved by simply increasing concentration.
An alternative approach to increasing cladding absorption would be to simply increase the area of the gain region; e.g., increase the diameter of the core region. However, when the diameter of the core region is increased too much, the capability of the fiber to remain operating in a single signal mode is adversely affected; that is, higher-order signal modes (HOMs) are allowed to propagate. The excitation of HOMs can create power instabilities that are highly undesirable and may damage the architecture of a fiber laser or amplifier. This constraint on the size of the core region also serves to limit undesirably the achievable mode-field diameter (MFD) of conventional DCFs.
Thus, there is a need to achieve increased cladding absorption in a double-clad, GPF while maintaining single-mode operation and without requiring increased concentration of the fiber's gain-producing species within the gain region.
There is also a need for a double-clad, GPF capable of supporting a MFD as large as approximately 16 μm.